PSU Problem - Mains Related?

Hello,

A couple weeks ago, I noticed a buzzing coming from my 750 Watt power supply. I didn't think much of it, so, I assumed it to be an issue of RFI/EMI on the line, and bought a power strip with noise suppression.

My issues are now worse. The power supply buzzes loudly when the video card (nVidia GTX 570) is under significant load. I assumed it to be a bad PSU, and filed an RMA with Corsair, my manufacturer. That power supply is now on the way to me.

Unfortunately, I now notice that my girlfriend's power supply off of the same outlet is also buzzing aggressively while under load. I live in an extremely old house, and have had suspicions of my outlets not putting out a consistent 60Hz. A frequency instability, according to the interweb, would be consistent with my issues.

So accordingly, my questions are thus:
Should I be looking into a line conditioner / UPS with line conditioning?
If I need a UPS, would a line-interactive UPS be sufficient, or do I need an Online / Double-Conversion unit?

Thanks!